<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <div id="app">
    <p>{{msg}}</p>
    <script>
      const vm = {}
      let val = 'hello'
      Object.defineProperty(vm,'msg',{
        get(){
          return val
        },
        set(newVal){
          document.querySelector('#app p').innerHTML = vm.msg
          val = newVal
        }
      })
      document.querySelector('#app p').innerHTML = vm.msg
 
    </script>
  </div>
</body>
</html>
